The preparation of copolymers of maleic anhydride with various unsaturated monomers, particularly ethylene and styrene, is well known in the art. One of the more common techniques for effecting the polymerization of such copolymers involves solution polymerization. Briefly, solution polymerization is a process in which the monomers and polymerization initiators are dissolved in a solvent or diluent at the beginning of the polymerization reaction; the solvent or diluent is usually also a solvent for the resulting copolymer. A problem that has persisted in the art is the development of a suitable method for precipitating the polymerized product from the polymerization solution. One commonly used technique is to precipitate the polymer in methanol. This technique has several disadvantages relating to the toxicity, flammability and cost of methanol. Additionally the resin produced using methanol generally has a fibrous texture, low bulk density and often exhibits handling problems such as, for example, bridging the extruder hoppers during pelletizing. Consequently, considerable effort has been expended to devise suitable and effective techniques for precipitating copolymers of maleic anhydride from polymerization solutions.
For example, U.S. Pat. No. 3,461,108 teaches the employment of certain solvents for the polymerization step, keeping the monomers and in some instances the polymers also in solution during polymerization and then adding to the resulting solution another liquid, which causes precipitation of the polymer products in finely divided filterable solid form. The selection of the combination of reaction solvent and precipitating solvent has been characterized as being critical; a limited choice of solvent combination appears to exist and no completely satisfactory precipitating solvent has been found for some copolymers (U.S. Pat. No. 3,729,450). Furthermore, the solvents tend to be costly and, therefore, must be separated and recovered. An alternate approach is disclosed in U.S. Pat. No. 3,729,450 wherein copolymers containing maleic anhydride are copolymerized in a solvent for both monomers and the resulting copolymers. The copolymer products are then recovered by steam distillation until solvent and other liquid materials are removed, leaving a molten copolymer which is then poured on a chilled metal surface to solidify the polymer. The disclosure indicates that the steam stripped copolymers are non-tacky and brittle so that they can be removed from the metal surface and broken into chunks by bending or inverting the chilled surface and allowing the solid material to fall a substantial distance into a container.
U.S. Pat. No. 4,145,375 teaches a process for copolymerizing styrene and maleic anhydride by the steps of gradually mixing maleic anhydride with styrene in a mass stage under polymerizing conditions to form styrene-maleic anhydride polymer and then suspending the styrene-rich mixture in water, the styrene polymerization is completed as in a conventional mass/suspension polymerization system. Following polymerization, polymeric beads are separated from the water and dried. The disclosure indicates that after drying, the beads are likely to contain internal moisture and in addition approximately 15% of the bound maleic anhydride may be present in the acid rather than anhydride form, and, consequently, during extrusion, the acid portion reverts to the anhydride with the elimination of water. The disclosure indicates that a well-vented extruder is needed to produce a bubble-free extrudate.
U.S. Pat. No. 3,646,044 discloses a process wherein maleic anhydride and a vinyl monomer, such as styrene, are copolymerized in an aqueous medium containing a water-miscible organic solvent and a free radical polymerization catalyst so that polymerization takes place in the aqueous medium and an acid copolymer precipitates as it is formed. The water miscible organic solvent becomes associated with the acid copolymer and settles therewith to form a separate, usually gummy, layer at the bottom. An upper aqueous layer is formed which contains very little resin solids.
The methods heretofore known for precipitating polymeric resins containing maleic anhydride from solution polymerization solvents are not comparably facile when compared to the process of the present invention. Likewise, the polymeric products obtained therefrom are attendant with problems not present in the products produced in accordance with the present invention. For example, the problems of selecting non-solvent materials that are suitable for the particular polymeric material to be precipitated that may, in any event, be toxic, flammable and/or costly are avoided with the present invention. Similarly, the problems of condensing and recovering distilled solvents are also avoided. The products obtained with most of the prior art methods exhibit handling problems (e.g., bridging in extruder hoppers) and processing problems (e.g., foamed extrudate products) that are substantially avoided with the present invention; the prior art methods require additional processing (e.g., heat treating) to overcome these problems that can be avoided or minimized with the present invention.